/Engine/Plugins/Protocols/MQTT/Source/MQTTCore/Public/MQTTProtocol.h
MQTT v3.1.1
enum EMQTTConnectReturnCode { Accepted = 0x00U, RefusedProtocolVersion = 0x01U, RefusedIdentifierRejected = 0x02U, RefusedServerUnavailable = 0x03U, RefusedBadUsernamePassword = 0x04U, RefusedNotAuthorized = 0x05U, AlreadyConnected, InvalidURL, SocketError, }